That's not what dark fiber is. Dark fiber is dedicated fiber optic strands that are not in use and available to be leased to a customer providing their own optical equipment.
Enterprises can outsource management of the capital equipment (wavelengths) or the entire network (e.g. Ethernet). The latter is known is lit services and is much more common than dark fiber.
No "unused" does not automatically imply "overbuilt." There are many companies such as Lightower that build a network of "route miles" with the sole intention of leasing fiber. This is not the same thing as selling excess capacity because of a miscalculation.
They could also use it for existing customer traffic, but don't, as a means to control the supply to control the price and win other political objectives.
